Widely spread positive working photosensitive lithographic printing plates comprise an aluminum substrate having thereon a positive working photosensitive layer containing an o-quinonediazide compound, in which the aluminum substrate has been subjected to surface graining, alkali etching or anodizing. The o-quinonediazide compound is known to be changeable to a corresponding carboxylic acid on exposure to ultraviolet light (hereafter referred to as "UV" for simplicity). After UV exposure, the photosensitive layer in only the UV-exposed area is dissolved away with an aqueous alkali solution to expose the substrate, leaving the lipophilic photosensitive layer on the unexposed area to form an ink-receptive image area. The UV-exposed area (non-image area) where a hydrophilic (water-wettable) surface of the substrate is exposed is to retain water to provide an ink-repellant area.
Because the aluminum substrate surface has insufficient water wettability, i.e., ink repellency, printing ink adheres thereto to stain the non-image area. In order to improve the stain resistance of the aluminum substrate, it is generally necessary to make the non-image area water-wettable. If a substrate is made water-wettable before a positive working photosensitive layer is provided thereon, the resulting printing plate will have a reduced press life (the number of satisfactory prints obtainable) on account of poor adhesion between the hydrophilic substrate and the lipophilic photosensitive layer. Therefore, it has been a generally followed practice that only the non-image area is rendered water-wettable by using a developer containing a silicate, such as sodium silicate or potassium silicate, at the time of plate development.
However, the silicate-containing developer precipitates solids derived from SiO.sub.2, or the waste of the developer forms gel derived from SiO.sub.2 on neutralization. As a result, solid matter mainly comprising silicates formed in a processing tank or pipes of an automatic processing machine tends to cause a sensor to operate incorrectly or clog the pipes. It has therefore been demanded to develop a technique for providing a positive working photosensitive layer on a substrate having been previously made water-wettable with good adhesion thereby producing a printing plate having a satisfactory press life.
In order to overcome the above-described problems, U.S. Pat. No. 3,136,636 proposes providing an intermediate layer of a water-soluble polymer, such as polyacrylic acid or carboxymethyl hydroxyethyl cellulose, but the impression capacity (press life) of the resulting plate is still unsatisfactory. U.S. Pat. No. 4,483,913 teaches providing an intermediate layer comprising a quaternary ammonium compound, such as poly(dimethyldiallylammonium chloride), but the resulting plate is unsatisfactory due to insufficient stain resistance.
It has also been pointed out that a positive working lithographic printing plate precursor comprising a substrate having been previously made water-wettable and a positive working photosensitive layer does not have sufficient deletion properties (ease in deleting the unnecessary image area with a so-called deletion fluid after development), and improvement in this respect has been desired. Further, the plate precursor tends to suffer from color remaining (the phenomenon that dyes present in the photosensitive layer remain on the substrate in the non-image area after development) or tends to have insufficient print-out properties (an ability of providing a difference of optical density between an exposed area and an unexposed area before development), and improvements in these respects have also been demanded.